The invention relates to polymer formulations for use in the fabrication of electro-optic devices. Such devices can include a variety of electro-optic media, including electrophoretic media, electrochromic media, light-emitting diode media, or liquid crystal media. In particular, the invention is useful for improving the performance of layered assemblies including electro-optic media that are incorporated into electro-optic displays, such as displays found in monitors, mobile devices, tablets, electronic readers, and signs. The invention is also useful for fabricating bulk materials having electrically-switchable optical characteristics, such as variable transmission media, which may be incorporated into windows, art, furniture, or other architectural products.
A front plane laminate (FPL) e.g., as described in U.S. Pat. No. 6,982,178, typically consists of at least a transparent electrode, an electro-optic medium, an adhesive layer and a release layer. Assembly of an electro-optic display using an FPL may be effected by removing the release sheet from the FPL and contacting the adhesive layer with a backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, layer of electro-optic medium and electrically-conductive layer to the backplane. See FIG. 1A. This process is well-adapted to mass production since the front plane laminate may be mass produced, typically using roll-to-roll coating techniques, and then cut into pieces of any size needed for use with specific backplanes. In some embodiments, the backplane is microfabricated to include an active matrix of transistors.
As an alternative to a “simple” FPL, as described above, U.S. Pat. No. 7,561,324 describes a so-called “double release sheet” which is essentially a simplified version of the front plane laminate of U.S. Pat. No. 6,982,178. One form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet. Another form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the double release film are intended for use in a process generally similar to the process for assembling an electro-optic display from a front plane laminate (FPL) already described, but involving two separate laminations; typically, in a first lamination the double release sheet is laminated to a front electrode to form a front sub-assembly, and then in a second lamination the front sub-assembly is laminated to a backplane to form the final display, although the order of these two laminations could be reversed if desired (See FIG. 1C).
As an alternative construction, U.S. Pat. No. 7,839,564 describes a so-called “inverted front plane laminate”, which is a variant of the front plane laminate described in U.S. Pat. No. 6,982,178. This inverted front plane laminate comprises, in order, at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer; an adhesive layer; a layer of a solid electro-optic medium; and a release sheet. See FIG. 1B. This inverted front plane laminate is used to form an electro-optic display having a layer of lamination adhesive between the electro-optic layer and the front electrode or front substrate; a second, typically thin layer of adhesive may or may not be present between the electro-optic layer and a backplane. Such electro-optic displays can combine good resolution with good low temperature performance. (See FIG. 1B.)
As discussed in U.S. Pat. Nos. 7,012,735 and 7,173,752, the selection of a lamination adhesive for use in an electro-optic display (or in a front plane laminate, inverted front plane laminate, double release film or other sub-assembly used to produce such an electro-optic display) presents certain problems. Because the lamination adhesive is normally located between the electrodes, which apply the electric field needed to change the electrical state of the electro-optic medium, the conductive properties of the adhesive can influence performance greatly.
For the most part, the volume resistivity of the lamination adhesive controls the overall voltage drop across the electro-optic medium, which is critical factor in the performance of the medium. [The voltage drop across the electro-optic medium is equal to the voltage drop across the electrodes, minus the voltage drop across the lamination adhesive.] On one hand, if the resistivity of the adhesive layer is too high, a substantial voltage drop will occur within the adhesive layer, requiring higher voltages between the electrodes to produce a working voltage drop at the electro-optic medium. Increasing the voltage across the electrodes in this manner is undesirable, however, because it increases power consumption, and may require the use of more complex and expensive control circuitry to produce and switch the increased voltages. On the other hand, if the resistivity of the adhesive layer is too low, there will be undesirable cross-talk between adjacent electrodes (i.e., active matrix electrodes) or the device may simply short out. Also, because the volume resistivity of most materials decreases rapidly with increasing temperature, if the volume resistivity of the adhesive is too low, the performance of the display will vary greatly with temperatures substantially above room temperature.
For these reasons, there is an optimum range of lamination adhesive resistivity values for use with most electro-optic media, this range varying with the resistivity of the electro-optic medium. The volume resistivities of encapsulated electrophoretic media are typically around 1010 Ohm cm, and the resistivities of other electro-optic media are usually of the same order of magnitude. Accordingly, the volume resistivity of the lamination adhesive should normally be around 108 to 1012 Ohm cm, or about 109 to 1011 Ohm cm, assuming an operating temperature of the display, typically around 20° C. Preferably, the lamination adhesive will also have a variation of volume resistivity with temperature that is similar to the electro-optic medium itself.
In addition to the electrical properties, the lamination adhesive must fulfill several mechanical and rheological criteria, including strength of adhesive, flexibility, ability to withstand and flow at lamination temperatures, etc. The number of commercially-available adhesives which can meet all the relevant electrical and mechanical criteria is small, and in practice, the most suitable lamination adhesives are certain polyurethanes, such as those described in U.S. Pat. No. 7,342,068.
To improve the performance of commercially-available polyurethanes, the polyurethanes can be doped with salts or other materials, e.g., as described in the aforementioned U.S. Pat. Nos. 7,012,735 and 7,173,752. A preferred dopant for this purpose is tetrabutylammonium hexafluorophosphate. With experience, it was discovered that adhesives formulated with such dopants damage active matrix backplanes, especially those including transistors made from organic semiconductors. U.S. Pat. No. 8,188,942 showed that, in some embodiments, the salt dopants could be replaced with polymer additives having hydroxyl groups (such as poly(ethylene glycol) to improve the volume resistivity of the adhesive formulations.
Unfortunately, polyurethane compositions with salt and/or polymeric additives have been found to form voids when applied to electro-optic media having irregular surfaces. To counteract the voids, thicker layers of adhesive are applied during the construction of an electro-optic assembly, e.g., a front plane laminate or display. The thicker layers discourage void formation and improve planarity between the electro-optic media and the electrodes. The improved planarity results in more consistent grayscales over the surface of a display, however, the increased thickness reduce sharpness in the display because the field lines between the backplane and front electrode are more diffuse. Copending application Ser. No. 14/692,854, filed Apr. 22, 2015, describes further efforts to improve the planarity of the adhesive layer while diminishing void formation. Copending application Ser. No. 14/692,854 also discloses substantially solvent-free polyurethane formulations that can be used to produce adhesive layers with improved hardness and durability.